The invention concerns a method for calibration of hydrophone groups, especially hydrophone groups in seismic streamers or sections of seismic streamers. The invention also concerns a calibration system for hydrophone groups, especially hydrophone groups in seismic streamers or sections of seismic streamers, wherein the calibration system comprises a signal analyzer connected respectively with a voltage source for a measuring microphone, a hydrophone group selector and a power amplifier for driving a sound source connected to the signal generator of the signal analyzer.
There is a well-known technique of calibrating the hydrophones in a seismic streamer individually by means of a hydrostatic pressure pulse in a fluid-filled chamber or similar arrangement. Such calibration systems are known from, among others, U.S. Pat. No. 4,320,468 (Montross), U.S. Pat. No. 4,375,679 (Park et al.) and U.S. Pat. No. 4,441,173 (McEachern). Alternatively, as shown in U.S. Pat. No. 4,563,756 (Assard), an acoustic signal simulator can be used. A method is also previously known of calibrating individual hydrophones by supplying them with electrical signals and measuring output voltages or currents. Such calibration systems are described, among others, in European application No. EP 203 227A1 (Quellhorst) or U.S. Pat. No. 4,648,078 (Darton et al.). Finally from U.S. Pat. No. 4,290,123 a method is known for calibrating the hydrophones in a hydrophone group by towing sections of a seismic streamer in the water past a small acoustic projector for acoustic test signals.
The present applicant has moreover for calibrating individual hydrophones, used a calibration system called "Distofon" which is based on the use of a hydrophone calibrator of type 4223 from Bruel & Kj r. With this system an accuracy of 0.5 dB has been achieved in calibrating the hydrophones. It permits calibration of hydrophones in the frequency range 30 to 300 Hz.
The present applicant has also developed a system for calibrating entire sections or hydrophone groups, called "KAVAC". This system is based on a description of G. Pickens and comprises six woofer loudspeakers mounted in the cover of a chamber with a volume of approximately 2.5 cubic meters. The calibration of hydrophone groups is performed in the "KAVAC" system at approximately 15 Hz, but it is encumbered with a variety of disadvantages, such as resonance in the chamber walls and insufficient sealing, which results in a dipole effect at low frequencies. This means that it is not possible to achieve a uniform sound field. Since the "KAVAC" system is also large and unwieldy, there is a much greater chance of obtaining incorrect measurements.
As far as the other above-mentioned calibration systems are concerned, none of these offers a rational solution to the problem of calibrating hydrophones in seismic streamers. They are mainly intended for calibration of hydrophones individually, a procedure which not only makes calibration time consuming and expensive, but also makes it difficult to attain consistent measuring conditions, thereby leading to inaccurate calibration.